With the recent explosive increase in network traffic, ultra-high-speed optical transmission systems of 40 Gbit/s, beyond 100 Gbit/s have been investigated. For such ultra-high-speed optical transmission systems, active investigation has been conducted on digital coherent communication combining a phase modulation method with coherent detection and digital signal processing technology, which is superior in characteristics required for long haul optical fiber transmission, such as tolerance characteristics against optical signal noise, chromatic dispersion, and polarization mode dispersion.
As a modulation method, Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK) have attracted attention because of their excellent dispersion compensation tolerance.
Further, in order to expand transmission capacity without increase in the frequency bandwidth, research and development have been extensively conducted toward the practical use on a method which is superior in frequency usage efficiency, such as Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK) in which QPSK signals are multiplexed by two orthogonal polarizations.
An optical receiver for digital coherent communication will be described below. The present description will be given using the QPSK method as an example. FIG. 9 is a functional block diagram of an optical receiver related to the present invention. With reference to this functional block diagram, a reception process in digital coherent communication will be described.
First, an optical receiver 900 receives a polarization-multiplexed optical signal. A polarization separation section 901 separates the received optical signal into an optical signal component having a first polarization state and that with a second polarization state perpendicular to the first polarization. The optical receiver 900 includes a local oscillator light source 902 which outputs local oscillator light with almost the same frequency as the optical frequency of the received optical signal. The separated two optical signal components and the local oscillator light are inputted into a 90-degree optical hybrid 903. The 90-degree optical hybrid 903 outputs a total of four optical signals including of real and imaginary components of each signal light component, which has polarization state parallel to respective one of two orthogonal polarization axes. The four optical signals are converted by an optical detector 904 into analog electrical signals, and are subsequently converted by an analog-to-digital converter 905 into digital electrical signals. These digital electrical signals are transformed by a re-sampling unit, which is not illustrated in the drawing, into digital electrical signals sampled at the symbol rate (also referred to as a baud rate) of the optical signals, and are subsequently inputted into a digital signal processing unit 906. The digital signal processing unit 906 has functions of wavelength dispersion compensation, polarization chromatic compensation, and phase noise and frequency deviation compensation. For example, as compensation for optical carrier wave frequency deviation and optical phase deviation, compensation is performed on a frequency deviation between the received optical signal and the local oscillator light and on an optical phase rotation due to a phase deviation, respectively. After that, each of the electrical signals is demodulated by a symbol discrimination unit 907 into a bit string sent by an optical transmitter. In this way, digital coherent detection in an ultra-high-speed optical communication system is realized.
Hereinafter, a description will be given in more detail of the above-mentioned polarization separation section and 90-degree optical hybrid. For convenience, a functional block having at least both functions of the polarization separation section and the 90-degree optical hybrid is referred to as an optical receiving unit. With regard to such an optical receiving unit, a study on its standardization has been conducted in the OIF (Optical Internetworking Forum), which is an industry organization for promoting high-speed data communication, and development of an optical receiving unit following the standard has been carried out.
There are various kinds of means for realizing such an optical receiving unit. For example, Non-patent Literature 1 describes an example of realizing an optical receiving unit with a combination of bulk elements. However, when thus realizing an optical receiving unit with a combination of bulk elements, it is difficult to adjust positional relationships between a plurality of bulk elements. It is because the adjustment requires, for example, that the optical axes of the plurality of bulk elements are aligned with each other. Consequently, a planar lightwave circuit is considered to be promising as a means without requiring such adjustment of positional relationships. For example, Non-patent Literature 2 discloses an example of realization of an optical receiving unit using a planar lightwave circuit. A part referred to as a PBS in Non-patent Literature 2 corresponds to the polarization separation section. A part referred to as a 90-OH corresponds to the 90-degree optical hybrid. The PBS described in Non-patent Literature 2 outputs light beams with different polarization states at different ports, by adjusting the birefringence of the arms constituting the PBS and thereby providing a phase difference π between the polarization states. By this way, good transmission characteristics are obtained, and polarization separation is accordingly realized.
However, this method requires highly precise adjustment of the birefringence of the arms constituting the PBS. To realize such highly precise birefringence adjustment, it is necessary to control the birefringence of the arms by using UV or heat. Accordingly, the process of controlling the birefringence becomes complicated, and it is difficult to reduce the cost.
Patent Literature 1 discloses an example of a configuration in which the complexity of the process of controlling the birefringence is resolved. In Patent Literature 1, disclosed is a configuration in which a groove is formed at a part of a planar lightwave circuit, and a photonic crystal chip is inserted in the groove in a manner to intersect a waveguide so as to enable the photonic crystal chip to function. With this configuration, a polarization separation function can be realized without the need for the process of controlling birefringence.